Impact of surface tension on microchannel two-phase flow.

摘要

Understanding the physics of microchannel two-phase flow is important for a broad variety of engineering applications. At the microscale, small Bond number, capillary number, and Weber number imply that the surface tension force dominates gravity, viscous force, and inertial force. Furthermore, in the confined space with complex geometry, such as porous media, the interaction between fluid phase and solid phase is of particular importance, and the surface hydrophobicity and the contact angle hysteresis effect play a significant role. In micro-devices such as a heat pipe and compact heat exchangers which involve the microchannel phase change process, the inter-phase mass transfer coupled with the interfacial force may further add to the complexity of the problem. This leads to many unique characteristics of microchannel two-phase flow.;The first component of this thesis develops a numerical model within the frame work of volume-fraction-method to simulate the contact angle hysteresis effect governing the microchannel two-phase flow. The model is validated against two engineering problems: The sidewall water injection in the microchannel, and the droplet detachment on a spinning plate. The comparison between model prediction and the experimental visualization shows that the incorporation of the contact angle hysteresis model can dramatically improve the accuracy of the volume-of-fraction simulation of the microchannel two-phase flow. The new model is also capable of capturing a number of physical phenomena governed the contact angle hysteresis effect, such as the instability of the slug transport in the microchannel.;The work is also dedicated to the development and validation of the capillary force model used for simulating the multiphase flow in porous with controlled hydrophobicity. The model is then applied to the simulation of boiling flow in the vapor-venting microchannel, which enables the phase separation and heat removal capacity enhancement. The simulation shows the vapor-escape process through the hydrophobic porous membrane in the vapor-venting channel as well as the dry-out process in the conventional channel. The benefits and problems of the vapor-venting microchannel design are clearly illustrated by predictions of flow pattern, pressure evolution, and temperature profile. The simulation tool shows a capability of guiding the design and optimization of the next generation vapor-venting micro heat exchanger.;Condensational microchannel two-phase flow is also dominated by the strong coupling between capillary force and phase change process. The last part of this thesis explores the impact of surface tension and channel hydrophobicity on the microchannel condensation. The small channel dimensions pose major challenges for experimental measurement. Here, a high speed imaging technique in conjunction with the interferometry is employed to study the flow pattern and construct the liquid-vapor interface in the hydrophilic microchannel of various dimensions. The measured exotic interface shape is compared with the prediction of a compact model allowing for the capillary-assisted liquid transfer effect. The agreement clearly shows the dominant effect of the surface tension on the condensational flow in microchannel. A 24-channel thermocouple array is developed to measure the local temperature distribution on the microchannel wall, from which the local heat flux distribution in the channel is constructed based on solving an inverse problem. The measurement indicates that channels with different hydrophobicities yield distinct heat flux profiles, a phenomenon attributed to the different heat transfer mechanisms dominating the hydrophobic and hydrophilic microchannels. The influence of channel dimension on the heat transfer characteristic is also investigated.